Historically, airbags have been coated with one or more layers of polymeric material to enhance their performance, for example, by preventing the unwanted permeation of air through the fabric and, to a lesser extent, by protecting the fabric from detriment due to exposure to hot gases used to inflate the airbags. Polychloroprene was the polymer of choice in the early development of coated airbags. However, it was subsequently discovered that, when exposed to heat, polychloroprene tends to degrade and to release the components of hydrochloric acid, thereby potentially introducing hazardous chemicals into the surrounding and degrading the fabric component. This degradation issue, coupled with the desire to decrease the folded size of the completed airbag by using less coating material, led to the almost universal replacement of polychloroprene with silicone-based materials for use as airbag coatings.
Newer designs for airbags, particularly those being placed in the sides of passenger compartments, have introduced the requirement that the bags hold pressure longer under use. The requirement of longer air retention times and the use of lower coating levels of silicone polymer have highlighted the effect that a naturally lubricating silicone coating will allow the yarns in the airbag fabric to shift when a sewn seam is stressed. This shifting may lead to leakage of the inflating gas through pores formed from the shifting yarns or, in drastic cases, may cause the seam to fail. Since the airbag must retain its integrity during a collision event in order to sufficiently protect the vehicle occupants, there is a great need to provide coatings that provide both effective air retention characteristics and sufficient restriction of yarn shifting for the airbag to function properly.
As mentioned above, in recent years, silicone coatings have been utilized to provide such desired permeability and strength characteristics. Most often, these properties have been achieved by applying a first layer of gas-retaining polymer (such as a silicone-containing polymer) to the fabric surface and by applying a second, protective layer over the first layer. The second, protective layer prevents the airbag coating from sticking to itself when folded and stored (a condition known as “blocking”) and also protects the first, gas-retaining layer from damage due to aging, abrasion, and the like. In most situations, the second layer also helps to minimize the burn rate of the airbag to achieve a passing score on the horizontal burn test mandated by Federal Motor Vehicle Safety Standard (FMVSS) 302.
Often, polymers including polyurethane, acrylics, and the like are used, either as components of a silicone layer (as in the case of blends, hybrids, or interpenetrating polymer networks) or as separate coating layers, perhaps with a silicone-containing layer. Efforts to create multi-component airbag coatings have previously focused on combining silicone with different polymers in the same polymer network. U.S. Pat. Nos. 6,348,543; 6,468,929; and 6,545,092, all to Parker, describe the production of an airbag coating made of a vinyl-containing polysiloxane cross-linked to, or admixed with, an ethylene-containing copolymer, such as ethylene methyl acrylate or ethylene vinyl acetate. In an alternate approach, described in U.S. Pat. No. 6,846,004 to Parker, a silicone polymer is combined with a copolymer of ethylene and at least one polar monomer in the presence of a volatile solvent and, optionally, a curing catalyst. Yet another approach, which is described in US Patent Application Publication No. 2005-0100692 to Parker, involves coating the airbag fabrics with the cross-linked reaction product of a vinyl-containing silicone and a copolymer having silicone and non-silicone substituents, which may or may not have terminal Si—H groups.
In the area of multiple-layered coating systems, U.S. Pat. Nos. 6,239,046 and 6,641,686, both to Veiga et al., describe the use of a two-layer airbag coating, where the fabric-contacting layer is an adhesive polyurethane and the top layer is an elastomeric polysiloxane. Another approach, described in U.S. Pat. No. 6,734,123 to Veiga et al., uses multiple layers of polyurethane as the airbag coating material. In this instance, layers of adhesive polyurethane and elastomeric polyurethane are employed to achieve the desired properties. Yet another multi-layer coating system is provided in U.S. Pat. No. 6,770,578 to Veiga, in which a prime coat of polyurethane is applied to an airbag fabric, followed by one or more layers of polymer film. Such polymer films are formed of polyurethane, polyamide, or polyolefin.
U.S. Pat. No. 6,177,365 and U.S. Pat. No. 6,177,366, both to Li, describe airbag coating compositions comprising at least two separate and distinct layers. The first layer (base coat), being in contact with the airbag surface, comprises a non-silicone composition of at least one coating material and provides excellent adhesion, excellent tensile strength, and lower cost than standard silicone materials. The second layer, being a coating for the first layer, provides excellent reinforcement and aging characteristics to prevent degradation of the first layer. Such a second layer (topcoat) is preferably a silicone material. This two-layer system permits excellent strength and aging properties to prevent seam combing at relatively low cost due to the inexpensive basecoat materials and the relatively low add-on weight required for the topcoat.
Airbag manufacturers have used these and other solutions to address the multiple problems associated with forming a suitable coating composition. Most importantly, the airbag coating needs to provide the necessary gas-retention properties to the airbag. Secondly, the coating needs to impart flame retardance to the airbag. Historically, this problem has been solved by incorporating flame retardant additives into the top layer(s) of the coating, since the incorporation of flame retardant additives into the fabric-contacting layer impairs gas retention. A third problem faced by manufacturers is that the coating compositions tend to stick to themselves, when the bags are folded and stored over long periods. This issue, known as “blocking”, may cause the airbag coating to adhere to itself and pull away from the airbag as it is deployed. Finally, yet another problem is the need for the airbag coating to be stable to aging, meaning that the coating will not degrade over time and in extreme conditions of heat and/or humidity.
To date, no airbag manufacturers have been able to solve these problems with a single coating layer, which would be advantageous in terms of raw material and manufacturing costs. The present urethane-based coating composition, which may be used as a monolithic coating layer for airbags, provides a solution to these issues.